Chair back



FIG. 1 is a front elevational view of a chair back, showing my new design, the broken lines are for environmental purposes only and form no part of the claimed design;

FIG. 2 is a rear elevational view thereof; and,

FIG. 3 is a right side view thereof, a view taken from the left side being a mirror image thereof. 

The ornamental design for a chair back, as shown and described. 